This invention concerns fuel injection for a vehicle engine particularly a pump-line-nozzle type system and pump which are commonly used with diesel engines. Such a pump commonly utilizes a delivery valve having a backflow prevention valve element and a pressure relief element for the fuel line leading to the fuel injector. In a preferred embodiment, the delivery valve device is positioned downstream of the pump's internal pumping components and upstream of each outlet leading to the high-pressure lines which are connected to individual fuel injectors for each cylinder of the engine. Each of the high-pressure lines supplies pressurized fuel to a fuel injector associated with a cylinder and combustion chamber. In response to an increase in fuel pressure to a set pressure level, the fuel injector is opened for spraying fuel into an associated combustion chamber. Correspondingly, when the pressure level decreases to a certain level, the fuel injector closes.
Typically in such systems, a provision is made to prevent backflow of fuel from the lines to the pump. Also, provision is made to slightly relieve fuel pressure in the line by a "delivery" valve so as to prevent "dribble" injection through the injector. The delivery valve defines a "retraction volume" to slightly relieve the line pressure. Thus, the delivery valve has two important functions: first, it prevents fuel backflow via a check valve or the like; and second, it provides a pressure reducing function for fuel upstream of the fuel injector by means of providing a "retraction volume" mechanism located upstream of the pumping elements. Backflow prevention is important because it desirably traps pressurized fuel in a high-pressure line leading to the fuel injector so the line never empties but yet retains a pressure level high enough to prevent the hot fuel from boiling in it. The line pressure reduction quickly reduces fuel pressure upstream of the fuel injector at the close of the injection cycle to prevent dribble of fuel from passing the injector and entering the combustion chamber.
The backflow prevention function is important to conserve system energy by maintaining a relatively high fuel pressure in the fuel lines downstream of the fuel injector prior to an injection cycle for a particular cylinder. Accordingly, when the injection cycle is initiated at the next injection cycle, the fuel pressure need only be slightly increased to a level exceeding the injector's set opening characteristic. Naturally, the backflow prevention traps pressurized fuel in the line leading to the injector and conserves energy for the next injection cycle but also increases the responsiveness of the injection system and allows use of a smaller capacity fuel pump. Another advantage is the elimination of creation of a low pressure or vacuum conditions in the fuel line which can result from an instantaneous reversal of the fuel flow. In addition, line-cavitation is prevented which tends to harm a fuel line by internal erosion creating debris arising from the erosion. This debris can damage the fuel injector's nozzle tip which has very small and high-precision clearances therein.
The delivery valve's second pressure relief function for the high pressure fuel line is important to more precisely control both the quantity and the timing of fuel injection to a combustion chamber. While it was previously noted that maintenance of fuel pressure in the line is desirable, the pressure level maintained in the line should be at a level insufficient to cause any significant fuel flow past the injector's valve. Specifically, at the end of a designated injection period, the fuel's pressure level in the high-pressure line can be above the designed opening pressure level of the fuel injector. This pressure level can undesirably allow a small quantity of fuel to flow past the injector and into the combustion chamber. The undesirable additional injection of fuel may continue until the pressure level falls below the closing characteristic or criteria of the particular fuel injector. This condition, known as after-injection, post-injection, or "dribble", introduces a quantity of fuel in excess of the ideal quantity and also the dribble flow occurs at a continuously-decreasing pressure usually resulting in very poor fuel atomization. The usual result is a formation of relatively large fuel droplets that do not have any appreciable energy and enter the cylinder or combustion chamber late in the engine's combustion cycle. The large droplets do not atomize and evaporate completely and the fuel is not efficient in associating and combining with oxygen. Typically, the fuel droplets barely pyrolyze and all too often exit the combustion chamber as smoke. Accordingly, this condition produces high soot emissions and limit the power potential of the engine.
The above mentioned pressure relief function of the delivery valve enhances fuel injector performance for precisely cutting-off injection of fuel at the desired time of the engine's cycle. This prevention of post-injection is achieved by artificially creating additional volume communicated to the line to slightly lower line pressure at a precise time that the injection cycle is supposed to cease. This additional line volume is called a "retraction volume". Pressurized fuel is allowed to expand into the retraction volume to relieve line fuel pressure to a level below that of the closing characteristic of the injector. In this fashion, after-injections are precluded and precise cut-off of injection at the nozzle are achieved which results in clean exhaust with an absence of smoke or soot. This not only reduces emissions, but, by cleaning the exhaust of visible smoke, permits more powerful operation of the engine so as to handle greater loads. Consequently, the mechanical efficiency of the engine is increased and the specific fuel consumption is decreased.
As with any liquid fuel system, it is necessary to keep fuel in a liquid state rather than a vapor state in order to maintain the hydraulic integrity of the system. Thus, in all cases, the "retraction volume" of the delivery mechanism is carefully designed and controlled to maintain line pressure at a high enough level to prevent fuel within the line from boiling due to the high-temperature environment surrounding the fuel system; and/or from getting into a vacuum condition, and cavitating caused by dynamic pressure spikes within the system.
The previously discussed delivery valve is used with and is a part of an injector pump of a known plunger-and-barrel type. Typically, such a pump receives periodic mechanical engine timed inputs through a camshaft and tappets. In such a pump, the basic elements of the pump assembly are typically disposed vertically and extend upward from the engine camshaft and tappets in the following order: plungers which are axially movable in a stationary barrel assembly which are activated by the camshaft and tappets; the delivery valve assembly; and outlets and high pressure lines leading to fuel injectors, one located at each of the engine's cylinders and combustion chambers. Activation of the plungers by the camshaft and tappets moves the plunger upward which pressurizes and pumps fuel. The fuel flows past the delivery valve before passing through an outlet fitting and into a high-pressure line. The fuel flows to a fuel injector which is opened in response to a fuel pressure level above a set opening pressure level characteristic for the injector.
Typically, a delivery valve has two basic parts: a housing with a cylinder bore formed therein; and a small movable member also called the stem member reciprocal in the housing. The stem's outside dimension or surface is closely fitted in the cylinder bore and is moveable therein in response to the pressure of fuel and a force exerted by a spring. The stem's outside surface is typically fluted which provides a close tolerance but low-friction sliding relationship with the cylindrical bore. The flutes also allow fuel to pass between the end portions of the stem as well as acting as guides while minimizing contact between the stem and the housing. A retraction volume mechanism is located at an upper end portion of the stem and a check-valve is located at either end portions of the stem. Specifically, the retraction volume chamber or cavity is formed between the stem and the housing by creation of a retraction-volume land, or control-surface on the stem and forming an adjacent groove in the stem. In an axial direction of the stem, the retraction-volume land typically has a small height dimension, typically only about 1 mm.
In a typical construction using a top-end type check valve, the stem has a conically shaped neck formed just downstream (above)the retraction volume land portion. The neck portion has a larger diameter than the end of the adjacent cylinder bore in the housing and the upper edge of the cylinder bore serves as a seat for the conically shaped portion. The seating relationship closes the delivery valve to prevent a backflow of fuel to the pump interior during the non-injection or dwell period for that particular cylinder. The closing of the delivery valve by engagement of the conically shaped portion and the seat is aided by operation of gravity on the stem portion as well as a downward force produced by a light return spring.
In operation of the pump, the pumping chamber located above the plunger is first filled with fuel allowed by positioning of the top edge portion of the plunger below the lower edge of the fuel feed-hole or inlet. Then upward movement of the plunger by the input from the engine's cam and tappet moves the top edge past the inlet to block flow out of the pumping chamber. Increased pressure of fuel in the pumping chamber lifts the stem and its check valve from a seat against the biasing force of the stem re turn spring. With additional plunger movement, fuel trapped in the pumping chamber passes to the lower part of the delivery valve through the fluted configuration but cannot yet pass into the line to the fuel injector until the lower control edge of the retraction-volume land clears the top edge of the delivery valve housing. Then fuel is passed into the line to the fuel injector until the lower helical control curface of the stem registers with and passes the lower edge of the fuel feed-hole or inlet at which time pressurized fuel is relieved into the inlet. At this point, pumping action ceases as the pressure in the pumping chamber, attaining a maximum of 20,000 psi in some cases, instantly drops to the fuel inlet pressure which is typically about 35-50 psi.
When the above described spilling of pressure back into the inlet feed-hole occurs, the fuel pressure in the line would remain high and continue to flow through the injector nozzle which is set to open at a pressure of 1500-3500 psi. This flow would continue until the line pressure was reduced to below the injector opening pressure. A certain volume of fuel must be accomodated to relieve this high line pressure so that the fuel injection through the nozzle stops when the pump pressurization ceases. The volume to be relived is a function of the line pressure, the length of the line, the internal and external diameter of the line, and the spring chamber volume at the bottom of the oulet fitting. For a typical large truck engine, this volume could be about 70-80 cubic mm. For a smaller four cylinder automobile engine, the volume could be about 30-35 cubic mm.
The retraction volume of the delivery valve provides a space or volume for receiving the necessary quantity of fuel to relieve the high line pressure to prevent flow after the end of the pumping cycle. This pressure relief prevents formation of smoke and soot caused by injection of excess fuel late in the combustion process. The retraction volume is created as the delivery valve stem moves downward and its lower control edge formed by the retraction-volume land passes by the upper edge of the delivery valve housing. The continues to form as the stem continues its downward movement until the conically shaped check valve seats in a closed position. Thus, a space is created by downward displacement of the retraction-volume land past the upper control edge of the delivery valve housing. This fuel "retracted" is not lost back to the pump but remains in the delivery system ready to be part of the fuel pressurized and pumped in the next injection cycle. In this way, it is possible to affect a sharp and clean cut-off of fuel injected at the nozzle without additional "dribble" of excess fuel and while still maintaining a certain residual level of fuel pressure in the line. The residual pressure is set to be high enough to avoid line cavitation and/or boiling (evaporation) of fuel. Also, the residual pressure is set to be sufficiently low enough so that natural pressure spikes that occur under dynamic pressure conditions do not reach a level sufficiant to instantaneously crack the injector nozzle open.
In practice, it is desirable to minimize the line volume as well as the volume in the delivery valve's return spring cavity as this reduces the necessary sizing requirement for the retraction volume as much as possible. Reducing the necessary retraction volume resultantly reduces travel requirements of the retraction-volume land. All of the above help to: increase the responsiveness; decrease the closing impact characteristics. overall, this increases the sealing integrity and the durability of the system.
In the second popular valve design, the conical check valve is positioned at the bottom of the stem and the retraction-volume land at the top portion of the stem. This permits a hollow central portion of the stem for receiving a large portion of the stem return spring. This allows a shorter spring retaining volume at its upper end portion which allows the vertical dimension of the assembly to be shortened. This promotes a lighter valve design which requires less material and less machining resulting in a more compact, lower weight pump assembly.
The movement of the pump's plunger is produced by input from a lobe of a camshaft. Specifically, a tappet is acted upon by the cam lobe to first cause upward movement of the plunger. This movement first causes a closing of an inlet feed-hole formed in the pump barrel. This feed hole is in fluid communication with the discharge portion of the fuel supply system. Thereafter, further upward movement of the plunger pressurizes fuel. The pressure of the fuel lifts the conical check valve off its seat against the force of the return spring and gravity. With additional upward movement of the plunger, fuel in the pumping chamber is displaced through the lower fluted part of the delivery valve but cannot pass into the discharge line until the stem is moved so that the lower control edge of the retraction-volume land clears the top edge of the delivery-valve housing. Depending on the diameter of the retraction-volume land, and the amount and pressure of the fuel being pumped, the lower control edge of the retraction-volume land is moved past the upper control-edge of the delivery-valve housing by a finite distance. The stem remains in this delivery position during the injection period until the period ends as the plunger "spills" (discharged). Fuel is "spilled" as a lower helical control surface of the plunger registers with, and then surpasses, the lower edge of the feed hole in the barrel.
Also, in the second design, the flutes are replaced by flats machined on the sides of the stem. In the second prior art embodiment, the smaller conical section at the lower portion is smaller and this saves considerable machining and material, thus reducing cost. Since different sections consume axial space of the stem, valves with many changes in machined sections, such as the first example, are axially longer than otherwise which forces the valve housing also to be longer-than-needed. This in turn also requires a longer (taller) pump housing and results in a heavier, taller, costlier pump.
What is needed is a lower, lighter, smaller valve design which requires less material and machining, and results in a lower-cost, shorter (in height), lighter pump, which also allows more installation flexibility and a better line-layout.